Lithium and lithium-ion secondary or rechargeable batteries have found use in certain applications such as in cellular phones, camcorders, and laptop computers, and even more recently, in larger power applications such as in electric vehicles and hybrid electric vehicles. It is preferred in these applications that the secondary batteries have the highest specific capacity possible but still provide safe operating conditions and good cycleability so that the high specific capacity is maintained in subsequent recharging and discharging cycles.
Although there are various constructions for secondary batteries, each construction includes a positive electrode (or cathode), a negative electrode (or anode), a separator that separates the cathode and anode, and an electrolyte in electrochemical communication with the cathode and anode. For secondary lithium batteries, lithium ions are transferred from the anode to the cathode through the electrolyte when the secondary battery is being discharged, i.e., used for its specific application. During the discharge process, electrons are collected from the anode and pass to the cathode through an external circuit. When the secondary battery is being charged or recharged, the lithium ions are transferred from the cathode to the anode through the electrolyte.
Historically, secondary lithium batteries were produced using non-lithiated compounds having high specific capacities such as TiS2, MoS2, MnO2 and V2O5, as the cathode active materials. These cathode active materials were coupled with a lithium metal anode. When the secondary battery was discharged, lithium ions were transferred from the lithium metal anode to the cathode through the electrolyte. Unfortunately, upon cycling, the lithium metal developed dendrites that ultimately caused unsafe conditions in the battery. As a result, the production of these types of secondary batteries was stopped in the early 1990's in favor of lithium-ion batteries.
Lithium-ion batteries typically use lithium metal oxides such as LiCoO2 and LiNiO2 as cathode active materials coupled with a carbon-based anode. In these batteries, the lithium dendrite formation on the anode is avoided, thereby making the battery safer. However, the lithium, the amount of which determines the battery capacity, is totally supplied from the cathode. This limits the choice of cathode active materials because the active materials must contain removable lithium. Also, delithiated products corresponding to LiCoO2 and LiNiO2 formed during charging (e.g. LixCoO2 and LixNiO2 where 0.4<x<1.0) and overcharging (i.e. LixCoO2 and LixNiO2 where x<0.4) are not stable. In particular, these delithiated products tend to react with the electrolyte and generate heat, which raises safety concerns.
Furthermore, new lithium-ion cells or batteries are initially in a discharged state. During the first charge of lithium-ion cell, lithium moves from the cathode material, such as LiCoO2 or LiNiO2, to the anode material, such as graphite. The lithium moving from the cathode to the anode reacts with an electrolyte material in the cell, causing the formation of a passivation film on the anode. The passivation film formed on the anode is a solid electrolyte interface or SEI. Upon subsequent discharge, the lithium consumed by the formation of the SEI is not returned to the cathode. This results in a lithium-ion cell having a smaller capacity compared to the initial charge capacity because some of the lithium has been consumed by the formation of the SEI. The consumption of the available lithium reduces the capacity of the lithium-ion cell. This phenomenon is called irreversible capacity and is known to consume about 10% to 20% of the capacity of a lithium ion cell. Thus, after the initial charge of a lithium-ion cell, the lithium-ion cell loses about 10% to 20% of its capacity.